-LRB- CNN -RRB- -- It 's thousands of square miles wide , virtually devoid of oxygen and it has been blamed for an increase in shark attacks : the Gulf of Mexico `` Dead Zone '' is getting bigger and forcing marine life -- including sharks - into shore .

Signs warn the public in an area harboring high bacteria near a drain at Will Rogers State Beach , California .

The zone has been caused by a flood of nutrients , such as agricultural fertilizers , which boost algae production in the sea . These growths consume huge amounts of oxygen creating a `` marine desert '' almost devoid of life .

The `` Dead Zone '' varies in size each year , but in 1999 it was 7,728 square miles -- that 's nearly the size of Delaware and Connecticut combined .

The huge size of the `` Dead Zone ' is due to the increase in nutrient pollution flowing down rivers , including the Mississippi , which is estimated to have risen threefold in the last fifty years as chemicals become more and more common on farms .

Environmentalists fear that the drive to radically increase the amount of corn-based biofuels produced in the U.S. from 15 billion gallons to 36 billion by 2022 could increase pollution in the Mississippi by 19 per cent .

But the problem is by no means limited to U.S. waters .

Similar `` Dead Zones '' are being discovered across the world and a major United Nations report in 2003 found that the number had doubled each decade since the 1960 's .

The UN report also warned that the number will continue to increase as intensive agriculture spreads around the world and that they are already having a significant impact on commercial fish stocks . All of this can come as quite a surprise .

Growing water demands , more pollutants

Think about pollution and you tend to imagine tall smoking chimneys or pipes pouring industrial effluent into our rivers and lakes . But the use of chemicals in agriculture is increasingly becoming a concern for environmentalists across the world .

Agriculture , including livestock and poultry farming , can be a source of a wide range of pollutants that find their way into our water supplies through run-off and leaching . This happens when rainfall exceeds the capacity of the ground and it flows into watercourses and groundwater supplies taking dissolved pollutants with it .

These can include sediment from eroded land , as well as phosphorus and nitrogen compounds from chemical fertilizers and animal waste , which can also harbor disease pathogens .

These pollutants can have a serious effect on water sources by depleting oxygen levels , stunting the growth of plants and even suffocating fish -- as in the Gulf of Mexico `` Dead Zone . ''

The concentration of pollutants can be particularly high in drought years , when heavy water demand can reduce the flow rate in rivers and cut their ability to dilute chemicals .

The effects of this can be acute in the developing world , where the pressure to feed a growing population combined with a low level of regulation can cause serious problems .

A huge increase in the amount of synthetic chemicals being used in the Philippines over recent years has caused substantial environmental damage to the country 's water supplies , according to a 2008 report by Greenpeace .

Between 1961 and 2005 fertilizer use in the Philippines increased by 1000 percent .

`` This model of agricultural growth is fatally flawed because of declining crop yields and massive environmental impacts , '' says Greenpeace campaigner Daniel Ocampo .

`` Aside from causing land degradation and losses in soil fertility , agrochemicals cause water pollution that directly and indirectly affects human health . ''

According to Greenpeace , analysis of groundwater in the Benguet and Bulacan provinces in the Philippines , found that 30 percent of tested wells had nitrates levels above the World Health Organization -LRB- WHO -RRB- drinking water safety limit .

The Philippine National Economic and Development Authority -LRB- NEDA -RRB- says that 37 per cent of water pollution in the country originates from agricultural practices .

As well as causing an increase in the algal blooms that can cause `` Dead Zones , '' agricultural nitrates have been identified as a factor in the growth of toxic `` red tide '' algae and high levels in drinking water can also pose a health risk to humans , especially children .

Agriculture 's impact on water pollution

Clearly there is a need to shift away from the current industrial agriculture system which promotes the reliance on agrochemicals while neglecting to consider their negative effects on human health , the environment , and the economy of local communities .

The problem is particularly serious in China , where a combination of rapid development , a growing population and intensive agriculture has led to widespread pollution and even water shortages .

Scientists studying information from monitoring stations have said that 44 per cent of Chinese rivers are polluted .

`` Many lakes and water courses contain an excess of nutrients and need treatment before they are suitable as freshwater sources , '' the Organization for Economic Cooperation and Development said in 2005 .

According to Chinese state officials 10 per cent of farm land in the country is polluted posing a `` severe threat '' to food sources , with excessive agricultural chemical use being blamed , along with industrial effluent and solid waste .

But controlling water pollution from agricultural run-off presents many challenges : it occurs over a wide area , goes across borders and often the source is hard to identify .

It also varies over time and can increase or decrease depending on changes in land-use and ownership .

Lax local law enforcement , corruption and inefficiency can all compound the problem .

Legislators around the world are calling for a closer integration of environmental and agricultural policies , and more help for farmers in managing potentially hazardous substances .

But many environmentalists see a solution with increased incentives for organic farming , which uses fewer chemicals and , argue groups such as the UK Soil Association , with no appreciable decrease in yields . But while the solution remains a matter of debate , the scale of the problem continues to grow and grow . E-mail to a friend

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Agricultural pollution has a caused a ` Dead Zone ' in the Gulf of Mexico

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Globally the number of Dead Zones has doubled every decade since the 1960 's

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The drive to increase biofuel production could increase pollution in rivers and seas

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Pesticide use rose 1000 percent in the Philippines between 1961 and 2005